The goal of this work is to provide a scientific basis for treating stiff-knee gait, a common movement abnormality among children with cerebral palsy. The rectus femoris transfer surgery is frequently performed to treat stiff-knee gait. In this surgery, the distal tendon of the rectus femoris is detached from the patella and reattached to one of several sites posterior to the knee. This surgery is thought to convert the muscle from knee extensor to a knee flexor thereby allowing the muscle to assist knee flexion during walking. However, the surgical outcomes are inconsistent, and the in vivo function of the transferred rectus femoris is unknown. We hypothesize that remodeling of the rectus femoris after surgery results in adhesions to surrounding tissues in some patients, which dramatically alters the postoperative function of the muscle and degrades the outcome of the surgery. This study will use biomechanical models, magnetic resonance imaging techniques, and experimental measurements on human subjects to evaluate the function of the rectus femoris following tendon transfer. Aim 1 will assess the potential of the rectus femoris to flex the knee after transfer. Biomechanical models, constructed from MR images, will provide quantitative descriptions of musculoskeletal geometry for 8-12 patients undergoing rectus femoris transfers to different surgical sites. This work will determine the differences in the knee flexion moment arm of the rectus femoris after transfer to different sites. Aim 2 will determine if forces at the proximal and distal tendons of the rectus femoris are equal after transfer, or if force at the distal tendon is disrupted after the surgery. Hip and knee moments, generated by selective stimulation of the rectus femoris, will be measured in subjects with stiff-knee gait. Tendon forces will be estimated by dividing the measured moments by the corresponding moment arms obtained from the MRI-based models. Aim 3 will use cine phase-contrast MR imaging to analyze and visualize the relative motions of the rectus femoris and the surrounding muscles in vivo. The extent of scar formation following rectus femoris transfer will be investigated, and the effects of adhesions on the mechanics of the muscle will be assessed. Aim 4 will examine how postoperative function of the rectus femoris influences knee motion during gait. The success of this work will result in better, more predictable treatment outcomes.